1. Field of the Invention
The present invention relates to an outer rotor type hybrid stepping motor, which is used, for example, for driving office automation machine and apparatus and for positioning factory automation use facilities.
2. Conventional Art
A stepping motor is usually used as a driving use motor and is generally used, in particular, for precise positioning use. Specifically, a hybrid type (herein below frequently abbreviated as HB type) stepping motor, which utilizes both magnet and reluctance torque, is used for the purpose of highly accurate positioning. For example, JP-A-2003-70222 discloses an HB type stepping motor.
In order to perform a highly accurate positioning with an HB type stepping motor, it is necessary to reduce a base step angle which represents a unit rotation angle when a unit pulse is input and in order to reduce the base step angle, it is necessary to increase number of teeth (number of poles) in the circumferential direction. On the other hand, there is a demand to reduce the size of the stepping motor. However, when the size of the motor is reduce, the diameter thereof has to be reduced, which prevents the number of teeth from increasing because of processing limitation, therefore, the above necessity and demand are in a trade off relationship.
When designing an HB type stepping motor, number of teeth Nr of a rotor, magneto motive force Urn of a magnet, number of turns AC of a winding, gap average magnetic flux density Bg, axial direction lamination thickness L and permeance ratio P1/P2 are determined by making use of the following formula (1) in a manner to increase the torque Tq under a determined physical scale and within the range of the material properties to be used;Tq∝Nr×Um×AC×Bg×L×P1/P2  (1)
However, since the permeance ratio P1/P2 increases, when increasing the number of teeth Nr, and further, the number of turns AC is substantially determined by the physical scale of the motor, if the magneto motive force Um is required to increase, the cost of the motor increases. For this reason, the usable range of the parameters was generally limited for the motor design.
Further, Author: Toshiba, Small Motor Study Group “Practical Motor Design Manual” (Publisher: Sogo Electronic Research Co., Published Dec. 25, 1992) discloses that a teeth width ratio in a range of 0.3-0.45 determined by tooth width Tw and teeth pitch τ is to be used and the permeance ratio P1/P2 can be determined by estimating the maximum permeance and the minimum permeance of the gap with reference to the value of the teeth width ratio, the teeth pitch τ and the gap width δ g. According to the disclosed method, the smaller the number of teeth is, the larger the permeance ratio P1/P2 can be achieved at the teeth width ratio of about 0.35.
The above conventional art can be used as one of design guidelines for HB type stepping motors, through which a motor having a certain level of performance can be designed. However, when such as an extreme increase of pole number and size reduction of a motor is required, a limited solution can be obtained within the design guideline. Further, an evaluation of an experimental model according to the guideline is difficult, because the gap width of this type of motor is extremely small less than 50 μm and the magnetic characteristics of the materials used vary significantly depending on production means, therefore, there was a problem which prevents a parameter survey to determine optimum design values.